Effect of Cr doping on the device quality of semi-insulating GaAs substrate material

Abstract

Semi-insulating GaAs substrates are being used increasingly for direct ion implantation and active layer epitaxial growth for the fabrication of microwave devices. The present paper describes the experiments performed to study Cr-doped and undoped GaAs substrate material and its effect on the electrical properties of epitaxially-grown layers and on the activation efficiency of the implanted layers. Before epitaxial growth and ion implantation, the Cr-doped and undoped substrates were evaluated for thermal conversion and change in leakage current by a bakeout at 800δC for 30 min. None of the substrates tested showed any type conversion. Leakage current measured by point probe increased from 1 to about 2 μA at 1000 V in Cr-doped substrates, but the leakage current remained unchanged at about 3 μA at 1000 V on heat cycling the undoped substrates. Epitaxial layers were grown. by VPE on (100)-oriented Cr-doped and undoped substrates simultaneously. Mobility measurements were made on epilayers between 1 to 10 μm thickness by Van der Pauw analysis. Change in net carrier concentration was also measured by C-V profiling. The activation of ion-implanted layers was investigated using 29Si ions with a dose of 3 × 10 12/cm 2, 120 keV energy. The activation efficiency was estimated by comparison with the standard LSS data. FET saturation current, which is proportional to the electrical activity of the implanted layer, was measured on both types of substrates using stripe geometry ohmic contacts. Details of the experimental results are presented. Change in mobility, carrier concentration, and compensation ratio near the epilayer-substrate interface, and lower activation of ion-implanted layers in Cr-doped substrates, are attributed to the presence of chromium.